1. Field of the Industrial Application
The present invention relates to printing wire driving devices in wire dot printers, and more particularly to a printing wire driving device in a wire dot printer which can perform accurate control with small power and has a large printing wire driving power.
2. Description of the Prior Art
With dot printers are used to print characters, symbols, etc. in dot form by striking printing wires. In this connection, a variety of printing wire driving devices have been proposed in the art.
One example of the conventional driving devices is shown in FIG. 1. In the driving device, its driving lever 100 is pivotally supported at one end, and the other end is coupled to a printing wire 101. The other end is further coupled to a spring 102 to return the lever 100. As the electromagnet 103 provided on the side of the printing wire 101 attract the driving lever 100, the printing wire 101 is caused to strike the platen 105 in the direction of the arrow, as a result of which a character is printed on the printing sheet p disposed between the platen and the printing ribbon 104.
FIG. 2 shows a second example of the conventional driving device. In the driving device, its driving lever 110 is pivotally supported at one end, and the other end is coupled to a printing wire 101. A spring 111 is connected to the driving lever 110 near the other end in order to urge the driving lever towards the platen 105, or in the printing wire driving direction. A permanent magnet 112 to attract the driving lever 110 is provided on the side of the driving lever 110 which is opposite to the side where the printing wire 101 is provided. A coil 113 is wound on the permanent magnet 112. The force of attraction of the permanent is cancelled out by energizing the coil 113, as a result of which the driving lever 110 is driven by the spring and the printing wire 101 is moved in the direction of the arrow in FIG. 2 to print a character on the printing sheet P.
In the driving device of FIG. 1, the printing wire 101 is driven by the electromagnet 103. In the driving device of FIG. 2, the printing wire is driven by the spring 111. Therefore, in these driving devices, the printing wire driving force is limited; that is, it is impossible to obtain a sufficiently large driving force. The large driving force may be obtained by using a large electromagnet 103 in the driving device of FIG. 1. In the case of the driving device of FIG. 2, the large driving force may be obtained by using a stronger driving spring 111 and a large permanent magnet 112. However, these methods are disadvantageous in that a mechanism for driving a plurality of printing wires becomes bulky.
As was described above, each of the above-described driving devices cannot provide a sufficiently large driving force, and therefore cannot perform an emboss printing operation. A wire dot printer having the driving device cannot be used, for instance, as a check writer. In addition, since the driving force is insufficient, characters cannot be printed on a plurality of printing sheets simultaneously.
In order to overcome these difficulties, a printing wire driving device which uses a rotary cam to obtain a large driving force has been proposed.
The conventional driving device is as shown in FIG. 3. A returning spring 121 is connected to one end of a driving lever 120 in order to urge the latter 120 in the axial direction. A printing wire 101 is connected to the other end of the driving lever in such a manner that the wire 101 is extended in the axial direction. A control electromagnet 122 is arranged near the driving lever 120 with a gap therebetween. When the control electromagnet 122 is energized according to a printing instruction, the driving lever 120 is attracted by the electromagnet 122, i.e., it is moved downwardly in the figure. As a result, a protruded piece 123 which is connected to the printing wire 101 is struck by a rotating cam 124, so that the printing wire 101 is jerked to the platen 105, in the direction of the arrow in the figure, to print the character on the printing sheet P.
In the driving device of FIG. 3, the printing wire 101 can be sufficiently driven by the driving force of the rotating cam 124. However, since the driving control is performed by causing the control electromagnet 122 to directly attract the driving lever 120, the electromagnet 122 should have a large force of attraction. As was described above, a large force is required for performing the driving control. Accordingly, it is rather difficult to positively perform the driving control according to the printing instruction, and it is unavoidable to employ a relatively bulky electromagnet as the control electromagnet 122.
Furthermore, since the gap is provided between the driving lever 120 and the control electromagnet 122, a time lag occurs in the driving control.